It is known that linear low density polyethylene (LLDPE) when treated with peroxide has an improved transparency for use in making thin polyethylene films. Furthermore, peroxide treated LLDPE can be used to make finished objects such as film under operating conditions used for the conversion of high pressure polyethylene while still maintaining the excellent mechanical properties of LLDPE such as: greater rigidity at equal density; superior hot mechanical strength; improved resistance to cracking under tension; less jamming when tubular film is being produced; and better stretchability and improved resistance to perforation in film form.
Specifically, a process is known for improving melt strength of LLDPE by pretreating the polymer in melt form at a temperature of at least 230.degree. C. in the presence of an organic peroxide for a period of time equal to at least three times the half-life of the organic peroxide at the melt temperature. Such a treatment improves bubble stability in blown film extrusion.
It is also known that processing characteristics of LLDPE may be improved by adding a chemically modified ethylene polymer to a base resin. The use of the modified polymer appears to improve the melt strength of the overall blend, providing greater bubble stability when blowing films from such blends.
As stated above, peroxide treated LLDPE is known to be effective when used for the fabrication of thin films. Such thin films are used to manufacture various products, especially products used in conventional food wrapping and packaging, such as grocery sacks, plastic bags, bread bags and trash bags. In all of these applications, the material manufactured from the peroxide treated LLDPE is a very thin, flimsy material.
Polyethylene is also used to fabricate thicker materials which have quite different applications than food or trash packaging. One polyethylene application which has assumed increased importance in recent years is its use for geomembranes. These are the sheets of material used to line areas of land where toxic waste is stored, leachate ponds, municipal landfills, and other such types of land containment areas. Conventionally, the bottoms and sides of these land areas are lined with high density polyethylene (HDPE) sheets having a thickness of 40-100 milliinches (mils). The material produced is strong, non-biodegradable, impermeable to air and moisture, and is effective as a barrier against leakage of the toxic waste, household chemicals and other such liquids into the soil and drinking water. However, when such material is used to cover the top of such an area, it is too rigid and lacks the capacity to stretch and form a leak-proof barrier around the top of the often irregularly shaped material underneath.
Pure LLDPE has not been used as a material for geomembrane applications because previous attempts to fabricate materials of the thickness necessary for such applications (i.e., in the range of 30-100 mils) have been unsuccessful. Attempts to extrude either blown or cast (flat) LLDPE sheet material at such a thickness range have been unsuccessful because the material simply will not hold together during processing.
A material has been needed for geomembrane applications which could be formed into thick sheets which would be strong, non-biodegradable, impermeable to air and moisture, but which would also have the capacity to stretch and change shape in order to provide an effective covering for a toxic waste storage area, leachate ponds, municipal landfills, and other such land containment areas.
The present invention provides such a material made from peroxide-treated LLDPE material formed to the specifications needed to provide cover and liners for geomembrane lined land areas where which toxic waste and household chemicals are dumped, slurry ponds, and other land containment applications. The present inventors has also successfully overcome previous failures to extrude in either cast or blown sheet from LLDPE materials having a thickness in the range of 30 to 100 mils.